Some of these replies are really naive, impractical and a bit frightening. You're talking about the social contracts which enable us to live and work together! They balance rights in imperfect ways (how can they be perfect and everyone agree?!) that average out as enabling society to function.
To me, it's pretty obvious. Every time any individual child exercises their "right" to challenge/refuse/question/ignore [delete as applicable!], another child loses their corresponding right to hear/focus/progress/enjoy [or equivalent].
As a case in point, in response to a few "He wasn't even allowed to explain..." posts...
Do you know how many kids will need to "explain" something in the first 10 minutes of an average lesson?
The teacher could listen to them all - to why #1, 2 and 3 don't have any pen or pencil (the story often starts at 7am that morning, and may feature the dog and baby bro, so settle in, listeners...), why #5 and 6's laptops aren't charged, why #7 haven't filled their water bottle in break, why #8 absolutely have to deviate from the seating plan and why #9 and 10 haven't done their homework. They could make the other 20 wait and listen, or set them a different activity to the one planned so they can engage (leaving the 5 learning support kids without the usual check-in they'd get at the start of a task).
After that, the teacher could even adjudicate on exactly why #1 and 7s' rationales made sense, whereas #2-6 were less convincing, and #9 and #10 needed to be revisited later (and, of course, deal with the explanations as to why their decision's wrong... and also hear the sudden interventions of #11 and 12, who overhears and - understandably - realises they're in a similar position to #1 and #7 so should assert their right to explain, too...)
Or they could refuse to engage with any of them (hoping angry parent calls and emails wouldn't get in the way of 3 hours' GCSE marking and the meeting to counsel little Sarah that afternoon).
Sure, if there's something worth fighting, a genuine injustice, challenge it - but do so in an appropriate way that doesn't force corresponding injustices on other students! Or that sets a precedent where your child's right to "explain" undermines the whole damn system necessary to support the majority.
You think the uniform rules are wrong from a financial POV? That's a great cause - take it up! But involving your kid any more than informing them you're doing it and to standby isn't productive or fair on them, other kids or their teachers.
You think the uniforms are wrong from a point of principle, though?! Well, you could encourage a righteous campaign of civil disobedience that disregards other parents' agreement with the policy and interferes with other kids. You could also insist the Head steps away from that safeguarding paperwork to explain the rationale behind the school's uniform policy to you, personally, in depth. Or you could just accept that living in a society means disagreement and compromise (and not bring up a kid to be more likely to queue jump on the basis that they know they're in more of a justified rush than the other randoms quietly waiting - or more likely to apply words like "fascist" to strict uniform rules so such words lose their meaning and impact when they're really needed to distinguish real horrors).